HP 8340b Operating Instructions

The above should be understood when using power sweep. When the POWER dBm display is repeat¬
ing the ENTRY DISPLAY, it indicates start power. If the POWER dBm display is indicating ALC level, it
shows the average power over the sweep. (The circuit is heavily filtered above 5 Hz.) Since the start
and stop dwell times are unequal, this averge reading has little meaning.
In a variety of situations it is possible to drive the ALC level below -20 dBm, such as putting DC into
the AM input, or when externally leveled. Since the internal detector is inaccurate at these levels, the
POWER dBm display would be very misleading. For that reason, and as a warning, the POWER dBm
display blanks at ALC levels below -22.0 dBm. This may occur at an output power of -22 dBm, -32
dBm, -42 dBm, etc., depending on the attenuator setting.
Decoupled Mode
In some applications it is advantageous to control the ALC and attenuator separately, achieving
combinations of settings which are not available in the coupled mode. Press [SHIFT] [PWR SWP] to
achieve decoupled mode, which causes “ATTN:_dB, ALC:__._dBM’’ to show in the
ENTRY DISPLAY. The ALC setting is entered via keypad or [KNOB], while the attenuator is incre¬
mented with the [STEP] keys. As before, the POWER dB display indicates the true, composite output
power. (Note: If an entry is made in decoupled mode, and subsequently [POWER LEVEL] is pressed,
the HP 8340B/41B reverts to coupled mode. It will set itself to the same power level, but the attenuator
and ALC settings may be different.)
One use of decoupled operation is power sweep, where the output power linearly tracks the sweep
voltage ramp. The HP 8340B/41B can generate power sweeps of up to 40 dB, depending on fre¬
quency. The power at the start of the sweep is set via [POWER LEVEL] (coupled operation) or
[SHIFT] [PWR SWP] (decoupled operation), and the sweep range (the amount the power increases
during the sweep) is entered by pressing [PWR SWP] followed by the desired dB sweep value. If the
sweep range entered exceeds the ALC range (stop power greater than maximum available power) the
UNLEVELED annunciator will light at the end of sweep. No warning is given at the time of entry. If the
start power is entered via the [POWER LEVEL] key, the ALC is set no lower than —10 dBm, limiting
available power sweep range to 30 dB at 4.5 GHz, or 11 dB at 26.5 GHz (HP 8340B). Using decoupled
mode and setting the ALC to —20 dBm gives an additional 10 dB of sweep range (although at —20
dBm, start power uncertainty is degraded by ±1 dB).
Decoupled mode is also useful when working with mixers. Figure 3-30A shows a hypothetical setup
where a HP 8340B/41B is providing the small signal to a mixer. The HP 8340B/41B output is —8 dBm,
which in coupled mode results in ALC = — 8 dBm, ATTN = 0 dB. The mixer is driven with an LO of
+10 dBm, and has LO to RF isolation of 15 dB. The resulting LO feedthrough of — 5 dBm enters the
HP 8340B/41B’s OUTPUT port, goes through the attenuator with no loss, and arrives at the internal
detector. Dependng on frequency, it is possible for most of this energy to enter the detector. Since the
detector responds to its total input power regardless of frequency, this excess energy causes the
leveling circuit to reduce its output. In this example the reverse power is actually larger than the ALC
level, which may result in the HP 8340B/41B output being shut off.
Figure 3-30B shows the same setup, with decoupled mode used to give a —8 dBm output: ALC =
+2 dBm, ATTN = —10 dB. The ALC is 10 dB higher, and the attenuator reduces the LO feedthrough
by 10 dB. Thus the detector sees +2 dBm desired signal versus a possible -15 dBm undesired one.
This 17 dB difference results in a maximum 0.1 dB shift in the HP 8340B/41B output level.
Reverse power is a problem with spectrum analyzers that do not have preselection capabability.
Some analyzers have as much as +5 dBm LO feedthrough coming out of their RF input, at some
frequencies. The effects of reverse power are less in the heterodyne band (.01 to 2.3 GHz) where the
power amplifier provides some broadband matching. Similarly, from 2.3 to 26.5 GHz, reverse power
that is within 10 MHz of the HP 8340B/41B’s frequency may be partially absorbed by the YIG filter. If
the frequency difference is small enough to be within the leveling loop bandwidth (typically 10 kHz CW,
200 kHz sweep or AM), the effect of reverse power is amplitude modulation of the HP 8340B/41B’s
output. The AM rate equals the difference in RF frequencies. Reverse power problems may be treated
by using the unleveled mode, as described below.
3-100
Operating Information
HP 8340B/41B

HP 8340b Specifications

General

Brand	HP
Model	8340b
Category	Inverter
Language	English